Bearing Loads for RSJ

[Alan9757]

Hello all

I wonder if anyone can give me some advice with regard to calculating and specifying bearings for an RSJ installation in a domestic dwelling.

I own a 1918 brick built house and am about to remove a chimney breast in a ground floor room on the party wall in the rear tenement. The left side of the chimney has a nice original dresser built in so I am aiming to retain the left wall of the stack and use it as a bearing for the RSJ. Being a mechanical engineer by training I have done the calc's to determine the imposed loads on the RSJ (which only need to support the floor above as there is no first floor stack) and have spec'ed the RSJ accordingly. However I have no idea how to assess the structural capability of the brick and mortar to support the RSJ (Bearing load 5.77kN).

If it negates the need to calc I could just remove the wall and replace it with concrete block masonry.

My council seem a bit too relaxed and are happy to give me the go ahead but 'we don't check your calc's'. But I don't want to be in the mire when I come to sell the house and surveyors get involved.

If anyone has an idea's how I can do the job right without the need for a structural engineer I'd appreciate it.

Thanks

Alan

 

[Static]

What size is the wall your bearing onto(both ends)? Also what quality is the brick/mortar? Also what size beam are you intending to use and what span?

 

[Alan9757]

Hi Static, thanks for your reply

I have done a bit more investigation since my posting and also analysed the situation a bit more. I now realise that whilst there is no fireplace above the chimney breast there is (of course - Doh!) a small first floor flue which escaped my attention because it cunningly also forms the door opening of a room upstairs. I'll get back to that!

The span of the beam (not including bearing lengths) is 2.1m.

Bricks and mortar, on excavation, are in pretty good shape. Bricks are hard and mortar has good adherence and requires a good whack with a bolster to chip it out.

I'm planning to use the left wall of the breast as a bearing pier for the beam, this is currently only a single leave of bricks so I'm expecting to need to add another and tie the two together with proprietary ties.All topped off with a good sized padstone providing a bearing length at the end of the RSJ of about 20mm.

The right side of the beam will bear on the left corner of the chimney breast in the adjacent room (the dividing wall has been removed - non structural). This (adjacent) fireplace has an upstairs chimney breast above and thus the flue for the ground floor fireplace used to (now blocked off) divert to the left to before passing into the room above. As such the left side of the ground floor stack (which will form my right hand beam bearing) is a healthy box of brick - albeit only one brick thickness.
I plan to rebuild the top part of this flue using high density concrete block and add a healthy padstone for good measure. The length of this bearing can be about 400mm.

Currently I have run calc's which indicated that an RSJ of 178x102x19 would very adequately do the job with sub 0.5mm deflections at the maximum. Since stating a 5.7kN bearing load (each end) I have read a bit more and revised my calcs (Increasing live load and safety factor - 2000N/M^2 and 1.6 respectively). So now I'm at a bearing load of 11kN each end.
HOWEVER (there always is isn't there!) I had ignored the presence of the previously mentioned upstairs flue/door frame. This sits atop the right hand side of the chimney breast, therefore mid span of the RSJ (it would wouldn't it!) And I have little idea of calculating the contribution of this to the loads. At the moment my approach would be to calculate the number of bricks and volume of mortar in this flue into the attic and add an allowance for the chimney (shared - party wall) and add this as a dead load.

Just to complicate things, my RSJ has to sit forward of the party wall because the upstairs joists were jointed at the forward edge of the chimney breast which they rest on - so I need to support at the position of this joint. Can I Support the upstairs flue just on its front edge or should I support right back to the party wall?

So, the things which I'm struggling with are as follows;

1) How do I determine masonry and padstone sizes in a manner which will stand scrutiny later on?Is there a good document (eg. Brit Standard) which I could read?

2) What is the best way to determine the static load from the upstairs flue?

Sorry it's been such al long one - I hope there are quick easy (for you)answers you can give.

Many thanks

Alan

 

[Static]

Always a pleasure to help,

Your overall loading seems a little light on the bearings for what you decribe, but your steel is only at about 20% stress so can probably take anything you throw at it. Use 2.5kN/m2 for the floor loading (assuming timber) and i would take about 2kN/m run for the brick loading of the chimney. So at a guess your joists upstairs are spanning about 4m and you probably have a 4-5m chimney stack to support ontop. Giving about 15kN/m UDL on the steel. Giving you about 15.7kN bearing.

Bearing calcs for masonry are in BS5628 Part 1, but if you put in a block sized mass concrete padstone (440lg x 215dp x 100wd) right end and a 215 cube pad the other you will be well in. Your probably better off rebuilding the end of the left hand wall in engineering brick into a 215sq pier.

I hope ive not missed anything

I would rebuild the flue in engineering brick rather than high density block, youll find it alot easier.

Provide a minimum of 200mm bearing onto the padstones.

Can I Support the upstairs flue just on its front edge or should I support right back to the party wall?

Would need to see a plan of what your doing to answer.